Magnetic tape having characterized magnetic layer and magnetic recording and reproducing device

1. A magnetic tape comprising: a non-magnetic support; and a magnetic layer including ferromagnetic powder and a binding agent on the non-magnetic support, wherein the absolute value ΔN of the difference between the refractive index Nxy measured regarding an in-plane direction of the magnetic layer and the refractive index Nz measured regarding a thickness direction of the magnetic layer is 0.25 to 0.40, the logarithmic decrement acquired by a pendulum viscoelasticity test performed regarding the surface of the magnetic layer is 0.010 to 0.050, and the logarithmic decrement on the magnetic layer side is determined by the following method: securing a measurement sample of the magnetic tape with the measurement surface, which is the surface on the magnetic layer side, facing upward on a substrate in a pendulum viscoelasticity tester; disposing a columnar cylinder edge which is 4 mm in diameter and equipped with a pendulum 13 g in weight on the measurement surface of the measurement sample such that the long axis direction of the columnar cylinder edge runs parallel to the longitudinal direction of the measurement sample; raising the surface temperature of the substrate on which the measurement sample has been positioned at a rate of less than or equal to 5° C./min up to 80° C.; inducing initial oscillation of the pendulum; monitoring the displacement of the pendulum while it is oscillating to obtain a displacement-time curve for a measurement interval of greater than or equal to 10 minutes; and obtaining the logarithmic decrement Δ from the following equation: Δ = ln ⁡ ( A 1 A 2 ) + ln ⁡ ( A 2 A 3 ) + … ⁢ ⁢ ln ⁡ ( A n A n + 1 ) n wherein the interval from one minimum displacement to the next minimum displacement is adopted as one wave period; the number of waves contained in the displacement-time curve during one measurement interval is denoted by n, the difference between the minimum displacement and the maximum displacement of the n th wave is denoted by An, and the logarithmic decrement is calculated using the difference between the next minimum displacement and maximum displacement of the n th wave (Δ n+1 in the above equation).

2. The magnetic tape according to claim 1 , wherein Nxy>Nz, and the difference Nxy−Nz between the refractive index Nxy and the refractive index Nz is 0.25 to 0.40.

3. The magnetic tape according to claim 1 , further comprising: a non-magnetic layer including a non-magnetic powder and a binding agent between the non-magnetic support and the magnetic layer.

4. The magnetic tape according to claim 1 , further comprising: a back coating layer including a non-magnetic powder and a binding agent on a surface of the non-magnetic support opposite to a surface provided with the magnetic layer.

5. A magnetic recording and reproducing device comprising: the magnetic tape according to claim 1 ; and a magnetic head.